Effect of different fillers on thermal conductivity, tribological properties of Polyamide 6

An influence of different filler types and filler content on the thermal and abrasive wear properties of polyamide-6 is investigated. Al2O3, MgO, two glass powders with different SiO2 contents, and natural zeolite powder were selected as fillers. The fillers individually were added to the polymer matrix in proportions of 50 and 70% by weight. A hybrid filler-containing composite was created by mixing PA6/70 wt% MgO and PA6/80 wt% zeolite. The results show that the thermal conductive enhancement factor is highest for PA6/70 wt% Al2O3 (145%) and PA6/hybrid fillers 75 wt% (92%). The Lewis-Nielsen and Reciprocity models agreed with the measured data with less than 26% deviation, except for the MgO-loaded composites. In the case of a hybrid composite, the additive model proves to be a good approximation. The abrasive effect of the different fillers was characterised by the volume loss of the steel pin using the pin-on-disc method. A new parameter is developed that considers the thermal conductivity enhancing effect of the fillers and their abrasive effect. In addition to ceramic fillers, aluminium-hydro-silicate, e.g. natural zeolite, and their mixtures offer new opportunities for the development of thermally conductive composites, as they are more economical to use in manufacturing processes.

www.nature.com/scientificreports/ min, the feeding speed of 19 1/min and the melting temperature of 250-280 °C. The diameter of the screw in the extruder machine was 20 mm, and the length/diameter ratio was 40. The fixed weight fractions of filler were 50, 70 wt.%. The number after the name of the filler indicates the weight percentage (PA6/MgO-50 means composition with 50 wt% MgO filler). To increase thermal conductivity, hybrid filler was also formed from the different available powders. Two polyamide composites were mixed by a 50-50% weight ratio: one containing 70 wt.% of MgO and one containing 80 wt.% of natural zeolite (hereinafter referred to as PA/MgO-70/Zeo80-50/50). Finally, the mechanically granulated reinforced thermally conductive PA6 composites were obtained. After this, they were dried at 75 °C for 8 h while the moisture content of the material fell below 0.05%. The granular composites were injection moulded to obtain specimens with standard flat shapes by a KM80-160C1 injection moulding machine (KraussMaffei Technologies GmbH. Germany).
Characterisation. The thermal conductivity of the composites was measured at room temperature by a C-Therm TCI (C-Therm Technologies Ltd. Canada) apparatus. The testing samples were 20 mm in diameter and 2 mm thick. The Shore D hardness measurements were measured by Zwick/Roell equipment. Ten hardness measurements per piece were performed, and a minimum of four pieces were examined.
The thus prepared composites were cryogenically fractured in liquid nitrogen. A micrograph of fractured surfaces was acquired by a Hitachi S4800 scanning electron microscope (SEM, Hitachi Ltd, Japan) equipped with a BRUKER AXS type energy dispersive X-Ray spectrometer (EDS, Bruker GmbH, Germany) and a Helios G4 PFIM-SEM (Thermo Fisher Scientific Inc., USA).
Non-isothermal analyses were carried out using a Netzsch STA 449 F3 Jupiter TG-DSC differential scanning calorimeter. About 20 mg of each sample were weighed quite accurately in the aluminum DSC pan, placed in the DSC cell, and then, it was heated from 28 to 300 °C at a rate of 10 °C/min under a purified argon atmosphere. Each sample was kept for 3 min at this temperature. After that, they were cooled to 100 °C at the same scanning rate. Finally, the samples were reheated to 280 °C. All the endothermic measurements were taken from the second heating scan of the samples to remove the previous thermal history, and the exothermic measurements were taken from the first cooling scan. The degree of crystalline (X c ) was calculated from melting enthalpy values using the following equation: where ΔH m is the melting enthalpy of the samples, and ΔH 0 m is the enthalpy value of melting of the 100% crystalline form of PA6 (240 J/g) 33,34 .
Wear tests of the PA with different fillers were performed under dry sliding conditions using a pin-on-disc tribometer (CSEM Instruments, Switzerland) with continuous rotation. The coefficient of friction was recorded during the tests. A peripheral speed of 0.6 mm/s speed was used and the sliding distances were 5000 and 7500 m. www.nature.com/scientificreports/ The pin was a steel ball (745 HV) 8 mm in diameter with a load of 10 N. The abrasive effect of the different fillers was characterized by the volume loss of the steel ball. The smaller diameter of the ellipse on the abraded surface of the ball was measured with an optical microscope. The volume of the missing spherical cap of the ball was calculated. The specific wear rate (W r ) was defined according to: where ΔV is the volumetric loss of the ball after sliding, S is the sliding distance, and F is the load. After the tests, the wear tracks were characterized with an optical and scanning electron microscope.

Results and discussion
Morphology and particle size distributions of fillers. The goal was to create an isotropic composite with good thermal conductivity and less tool wear, so the morphology of the fillers was examined first. As can be seen in Fig. 1a-c, Al 2 O 3 and Fritt1-2 particles have an irregular shape with sharp fracture edges. For all three powders, based on the particle size distribution (Fig. 2a), the particle sizes are below 100 µm and the distributions show near-normal. The median (D 50 ) cumulative particle size is 25.14 µm, 15.05 µm, and 16.63 µm in the case of Al 2 O 3 , Fritt1, and Fritt2, respectively. By contrast, natural zeolite, and especially MgO, form larger and nearly spherical aggregates (Fig. 1d,e). Apparently, the aggregates have no sharp edges. MgO powder aggregates very quickly. The particle size distribution confirms the presence of aggregates in the case of MgO powder (Fig. 2b). Half of the volume of MgO consists of particles below 10 µm, while the other half consists of particles of 300-500 µm. The aggregates for MgO and natural zeolite powders comprised 1-30 µm plate-shaped pieces with a thickness of less than 1 micron (Fig. 1f). Natural zeolite consists of 45 wt% smectite, which has a layered structure. These layers are stacked together by weak van der Waals forces to form a clay particle 35 . This structure allows nanocomposites to be created using a clay mineral filler 35 . The median cumulative particle size of MgO and natural zeolite is 33.90 µm and 28.56 µm, respectively.

Microstructure of composites.
The cryo-fractured surfaces of composites with 70 wt.% filler content were investigated to determine the distribution state of fillers in the PA6 matrix. EDS elemental mappings of the composite cross-sections show the location of the filler particles in the matrix even more spectacularly. The zeolite, Al 2 O 3 , Fritt1, and Fritt2 filler particles exhibited relatively good dispersion into the PA6 phase (see Fig. 3a,b,d). In the case of 70 wt% filler content, the filler volume content of all PA6 composites is about 50% except Al 2 O 3, where 43%. With such a large amount of the second phase, it can be assumed that a thermal conduction pathway forms. A gap thinner than a micron is visible around some filler particles. The adhesion force was lower than the shrinkage force during the curing process. In all cases, the gap between the filler and  www.nature.com/scientificreports/ the matrix impairs thermal conductivity. In the case of Al 2 O 3 , Fritt1, and Fritt2 filler particles, one can observe two characteristics of the filler/PA6 composites fracture surface: (I) few particles pull out from the matrix, and some holes appeare; (II) debonding of the filler particles/PA6 interface, the direction of crack propagation was deflected by the particle and further propagated at the particle/matrix interface. Natural zeolite is well dispersed in the PA6 matrix; in fact, it appears that the zeolite content is more than 57.22 V/V%. Zeolite particles are composed of micro-nano scale layers, as can be seen in Fig. 1e, which probably break down into individual particles to form micro-nano-sized filler. The amount of MgO is much less than expected in the cross-sections studied (Fig. 3c). This is probably because MgO is highly agglomerated, and its distribution is not homogeneous.
In the case of hybrid filler (PA6/Mg70-PA6Zeo80-50/50), smaller MgO particles fill the volume between the larger zeolite particles (see Fig. 3e) and form a higher packing density of the fillers in the matrix. It is worth noting that, in this case, the distribution of MgO particles is homogeneous; they may have disintegrated during the mixing of the two types of composites. Due to the higher volume content of the fillers particles, which is 62%, the distance and resistance among adjacent conductive fillers decreased, and the thermal conductivity can increase. Therefore, the formation of more effective conductive pathways or networks in the matrix has great importance for enhancing thermal conductivity.

Hardness of composites.
The mechanical property, such as surface hardness (Shore D) of PA6 composites as a function of volume concentration (which is calculated from the material density) is shown in Fig. 4. As can be seen, the surface hardness continuously increases with increasing filler content. In the case of 50 wt%, the composite containing Al 2 O 3 has the lowest hardness (79 ShoreD), while the composite containing zeolite has the highest (83 ShoreD). In the case of 70 wt% filler content, also the Al 2 O 3 composite has the lowest hardness, while the zeolite composite has the highest (88.5). This might be a surprising result, since Al 2 O 3 filler has the highest hardness among the investigated fillers, and zeolite is almost half as hard. However, the difference in density between fillers must be taken into account. The density of the zeolite is lower than that of Al 2 O 3 , and because of this, the composite with zeolite content has a larger volume of filler at the same weight percentage (Fig. 4b) so that a higher hardness can be achieved than in the case of Al 2 O 3 . This makes it possible to produce especially hard composite (88 ShoreD), even if a filler with a lower hardness than alumina's hardness is used.
Thermal properties. The melting and nonisothermal crystallisation behaviours of the samples were comparatively investigated using DSC, and the results are presented in Fig. 5 and Table 4. Here, a second heating scan was considered to eliminate the previous thermal history of the composites. The onset crystallisation temperature (T c-onset ), crystallisation peak temperature (T c ), enthalpy of crystallisation (ΔH c ), melting peak temperature (T m ), enthalpy of melting (ΔH m ), and degree of crystallinity (X c ), are summarised in Table 4. The stable crystalline form of PA6 polymer is the monoclinic α-form crystal, which generally crystallises at T c > 150°C 36 . The pseudo-hexagonal γ-form crystals can crystalise in the quiescent melt at T c < 150 36,37 . In the case of the pure PA6 sample, a broad and high endothermic complex peak can be seen at T m = 223.6 °C (Fig. 5a). During melting, two processes take place, supported by the shoulder in the lower temperature part of the peak (≈ 215 °C) and the first derivative of the process. The different types of fillers modified the melting process of PA6. The peak temperatures shifted towards lower temperatures for all fillers (Table 4), and the shoulders became more dominant (Fig. 5a). Based on these phenomena, two kinds of crystallites, either of the same crystal form but having two different crystal thickness or of a different kind, namely α-and γ-form crystals melting process takes place. It is known from literature that, the melting peak temperature of α-form (≈ 220 °C) of PA6 is higher than γ-form (≈ 212 °C) 38 , glass 41 and montmorillonite 38 fillers promote γ-form crystallisation. Based on the literature data, this suggests that, in our case, too, γ-form crystals are present in the composites in addition to α-form. Increasing filler content from 50 to 70 wt% does not significantly change the melting peak temperature. However, the shoulder is becoming more pronounced, which indicates an increase in the amount of γ-form crystal. The maximum depression in T m for initial PA6 (223.6 °C) is observed for Zeo-80 filler (213.4 °C). The enthalpy of melting and the crystalline volume fraction for each filler follow the same trend and decrease with www.nature.com/scientificreports/ increasing the filler content. The reduction of the crystalline fraction can be due to several reasons; on the one hand, the fillers increase the viscosity of the melt, and on the other hand, the fillers improve nucleations but increasing filler content restricts crystal growth 37 . Figure 5b shows the exothermic crystallisation for different filler types. In the case of 50 wt% filler content, the crystallisation starts (T c-onset ) at a significantly higher temperature due to the addition of different filler types than in pure PA6 sample (190.3 °C), clearly indicating an   www.nature.com/scientificreports/ excellent heterogeneous nucleation effect of these filler types on the crystallisation of PA6 matrix. In the case of 50 wt% filler content, Al 2 O 3 has the highest nucleation capacity since its T c-onset is 195.1 °C, whereas the pure PA6 is 190.3 °C. Zeolite also has a high nucleation capacity, while Fritt1, Fritt2, and MgO fillers have slightly less. The crystallisation peak temperature (T c ) also increases due to the influence of 50 wt% Al 2 O 3 , Fritt1, Fritt2, MgO, and natural zeolite. Increasing the filler content from 50 to 70 (and 80 wt% in the case of zeolite), the start crystallisation temperature (T c-onset ) and crystallisation temperature (T c ) are reduced to a minimal extent. Crystallisation behaves differently under the influence of MgO-Zeo hybrid filler. When MgO-Zeo hybrid filler is added, both the onset temperature and the peak temperature of the crystallisation are nearly the same as for the pure PA6 sample. But it is worth mentioning that the crystallization process depends not only on nucleation and growth but also on viscosity.
Thermal conductivity of the composite. The thermal conductivity of all investigated fillers is higher than the PA6 matrix (Table 3). Al 2 O 3 has the highest thermal conductivity (36 W/mK), and MgO has slightly less conductivity (30 W/mK). The thermal conductivity of Fritt1, Fritt2, and natural zeolite is thirty times lower than that of MgO. The thermal conductivity of zeolites usually varies from 0.6 W/m K to almost 4 W/m K depending on the mine where the zeolite originated. Schnell et al. 42 found a thermal conductivity of 1.2 W/m K for zeolite with a sodalite type framework by non-equilibrium molecular dynamics (NEMD) simulation. The thermal conductivity of the samples as a function of the weight fraction of the fillers is shown in Fig. 6a and in Table 3. Every point of thermal conductivity is an average of three sample measurements. As can be seen, the conductivity of composites increases with the increase of the filler fraction. When the filler content is less than 50 wt%, the thermal conductivity increases slowly in the case of all the fillers. At 50% by weight, the percentage by volume of fillers varied between 24% (Al 2 O 3 ) and 36% (zeolite). The heat-conductive particles generally surrounded by a polymer matrix cannot form a continuous path at low loading. The isolated particles do not significantly affect the enhancement of thermal conductivity. The heat transfer between the matrix and the filler occurs according to a serial model, and the thermal conductivity increases very slowly due to high thermal contact resistance. As the concentration of the filler increases, particles begin to touch each other to form a more compact packing structure. As the amount of filler increases, the distance between the filler particles decreases, eventually creating a network of filler contacts. So, thermal conductivity grows significantly because of reducing thermal contact resistance, and the tendencies are similar in Ref. 43,44 . Zijin Lin et al. 45 proposed a factor (φ) that represents the variation in the thermal conductivity of the composites compared to the matrix. It was defined as follows: where λ, and λ p are composite and matrix thermal conductivity, respectively. As was expected, the highest thermal conductivity of Al 2 O 3 increases the thermal conductivity of polyamide to the greatest extent. The thermal conductivity of the polymer does not increase to the expected extent in the case of MgO filler due to the uneven distribution of MgO particles. Fritt1, Fritt2, and zeolite have almost the same thermal conductivity value, but they demonstrate different thermal enhancement effects (Fig. 6b). As shown in Fig. 6b, natural zeolite is more effective in increasing the thermal conductivity of composite compared to Fritt1 and Fritt2. The average grain size of zeolite and MgO is approximately half that of Al 2 O 3 , Fritt1, and Fritt2. Thus, for a composite of equal mass and weight percent charge, taking into account the densities, significantly more particles are in the composite with natural zeolite or MgO than in the composites containing Al 2 O 3 , Fritt1, and Fritt2. The many zeolite particles increase the potential for the formation of heat conduction pathways. In addition, natural zeolite particles contain very thin layers, which can be easily displaced due to weak van der Waals bonding force, and thus nano-sized layers www.nature.com/scientificreports/ can be formed during mixing in the composite. Increasing the filler loading leads to more filler-filler interfaces. The interfacial thermal resistance (ITR) value for the PA6/ natural zeolite interface is probably smaller than the ITR for PA6/Fritt1 and PA6/Fritt2 interfaces. The advantage of a hybrid filler system can be observed in Fig. 6. Two composites (containing PA/MgO-70/ Zeo80-50/50) were mixed, and the thermal conductivity was 0.94 W/mK. In this mixture composite, the volume content of MgO is 27.46%, while zeolite is 34.69; namely, 62.15 V/V% is the total filler content. Separately the thermal conductivity of the MgO-containing composite is 0.85 W/mK, and that of the zeolite-containing composite is equal to 0.75 W/mK, while when mixed in 50-50 wt.%, the thermal conductivity reached 0.94 W/mK. The mixing of two composites with lower thermal conductivity resulted in higher thermal conductivity. This increase could only be achieved by creating more thermal bridges in the composite. Namely, the gaps between the zeolite particles were filled with MgO particles, and this was confirmed by microscopic examination. The value of the interfacial thermal resistance between zeolite and MgO particles is lower than that between PA6 and any of the fillers. The thermal conductivity of the polymer composite is doubled by using a MgO-zeolite hybrid filler. The price of natural zeolite is much lower than synthetic MgO, so the use of zeolite also has an economic advantage.
Thermal conductivity models. There are numerous theoretical and empirical correlations in the literature for calculating the thermal conductivity of solid filler polymers. The simplest models of two-phase systems are the Parallel and Series models 46  Lewis and Nielsen's model 49 takes into account the shape and orientation of the filler: The values of A and ɸ m for many geometric shapes and orientations are given. (In our calculation A = 1.5 -spherical shape, ɸ m = 0.82-maximum packing fractions for uniaxial random arrangements.) The Effective Medium Theory (EMT) model is an implicit form and it considers other forms for spherical and ellipsoidal inclusions for conductivity 50 . It tackles materials with a completely random distribution of all the particles.
The Reciprocity model 51 assumes that a microstructure of two components remains statistically equivalent when exchanging the volume fractions of the components: where α = f / p . The Bruggeman model 52 generally is applicable for higher filler content. This is the implicit form: The experimental data are compared with the results predicted by theoretical models. Table 5 lists the values of the experimental data and the thermal conductivities calculated by different models. In our experiment, the smallest volume filler content is 24%. The most significant difference between the calculated and experimental values is found for high thermal conductivity Al 2 O 3 (Fig. 7a) and MgO filler in the parallel model. One reason for this is probably that the volume fraction of Al 2 O 3 filler was the smallest and no proper heat dissipation pathway was formed. In the case of MgO, the filler distribution was not uniform. Lewis-Nielsen's model gives the best approximation for Al 2 O 3 , Fritt1, Fritt2, and natural zeolite. The maximum deviation for these materials was 26%. Good agreement was found by the Reciprocity model for Fritt1, Fritt2, and natural zeolite. The maximum error is 25%. Observing the structure of the composites (Fig. 3b,d), the distribution of these fillers in the composites is indeed the closest to the Reciprocity model. Despite the simplicity of the Series model, its predictions give the best approximation of the experimental results for MgO filler content composite (Fig. 3c, Table 5). In the case of zeolite filler (Fig. 7 b), up to 50 wt%, the error of the Maxwell-Eucken model is less than 5%, but above 50% weight fraction, it is already more than 30% higher than the measured value. At our concentrations, the values calculated by the other models are much higher than the experimental values except in two cases (Series model-Al 2 O 3 -50 wt%, Maxwell-Eucken model natural zeolite 50 wt%). There may be several reasons for this discrepancy. The higher calculated thermal conductivity may be attributed to the fact, that these models cannot consider the high interfacial thermal resistance and the thin gap between the filler and the polymer matrix. There are several models for hybrid filler. In the additive approach by Spencer et al. 53 , the individual contributions of the two fillers are summed without double counting the contribution of the matrix: where λ add is the thermal conductivity of hybrid filler composite using two types of filler. λ f1 and λ f2 are the composite thermal conductivity, with f 1 and f 2 containing fillers, respectively.
Woodside-Messmer 54 proposed a quadratic parallel model for a two-phase hybrid composite: where λ f1 and λ f2 are the thermal conductivity of f 1 and f 2 fillers, respectively. V p , V f1 and V f2 are the volume fraction of the matrix, f 1 , and f 2 fillers. Using these two models for MgO-zeolite hybrid filler composite, the thermal conductivity is 1.11 and 2.15 W/ mK according to the additive and quadratic parallel model. The measured thermal conductivity of the hybrid filler is 0.94 W/mK. The additive approach proves to be a good approximation. Figure 8 represents the friction coefficient traces recorded during the pin-on-disc tests of five differentPA6-based composite specimens. Similar observations were also recorded for other samples. During the initial wear stage, the friction coefficient curves increase rapidly. After a certain distance, a steady state friction coefficient was obtained after a run-in of about 2000 m. However, it is apparent that the natural zeolite-filled composite exhibits pronounced fluctuations in friction coefficient throughout the test until 2000 m is reached. Between 2000 and 7500 m sliding distance, the zeolite-filled composite has the highest friction coefficient value (0.31 ± 0.02) and the hybrid-filled composite has the lowest friction coefficient (µ:0.16 ± 0.004). The friction coefficient of MgO, Fritt1, and Al 2 O 3 filled composites are 0.23 ± 0.01, 0.25 ± 0.01, and 0.26 ± 0.02, respectively. Observing the worn surface of the Al 2 O 3 -70 filled composite (Fig. 9a) it can be stated that debris are present on the worn surface, which supposes the formation of adhesive wear. The higher magnification SEM image (Fig. 9b) clearly shows that the Al 2 O 3 grain and the PA6 matrix have separated due to the tensile stress. The edges of the Al 2 O 3 grains on the surface produce a strong cutting effect. In the case of natural zeolite-filled composite, the particles of average particle diameter are partially or completely covered by the polymer matrix. Thin flakes have separated from the surface. Definite cutting edges are not visible during abrasion (Fig. 9c), the filler particles are not broken, but cracks are created between the matrix and the grains due to tensile stress, and then fall out of the matrix (Fig. 9d).

Abrasion wear test.
W r is the tool wear during the composite production estimated by the volume loss of the steel ball. The ball's wear rate characterises the composite's abrasive effect in our case. A smaller wear rate of the steel ball means a smaller abrasive effect on the composition. Figure 10a depicts the variation of specific wear rates with an increase in different filler weight percentages in PA6. As the reinforcement of filler is increased, a general development trend in the wear rate of the ball on the composite materials is observed. Comparing the abrasive effects of all five types of composites, it is found that the abrasive rate value for natural zeolite reinforced composite is the lowest, followed by Fritt1, Fritt2, MgO, and Al 2 O 3 . There is no linear relationship between the abrasive effect and the hardness of the composite, because the hardest composite was the 70 wt% natural zeolite composite. In comparison, the least hard composite was the 50 wt% Al 2 O 3 composite. However, if you look at the hardness of the fillers, the Al 2 O 3 filler has the highest hardness, followed by MgO, and then nearly equal hardness for (12) add   (Table 3). Mohs hardness of a steel is about 4. So it is understandable that Al 2 O 3 and MgO grains, when they are brought to the surface during abrasion, will greatly increase the wear of the softer steel tool. Similar to the thermal enhancement factor proposed by Lin et al. 45 , we have introduced an abrasive effect factor = wear rate enhancement factor that shows by what percentage the abrasive effect of the polymer has increased as an effect of the filler content: In the above equation W r,c and W r,m are the specific wear rate of composite and polymer matrix, respectively. The abrasive effects of the composites with Al 2 O 3 and MgO filler were more than 100 times higher than that of PA6 with hybrid filler or natural zeolite filler (Fig. 10b). The advantage of the hybrid filler is also reflected in the abrasive effect, as the PA/MgO-70/Zeo80-50/50composite contains 35 wt% MgO and 40 wt% natural zeolites, the strong abrasive effect of MgO is significantly reduced.
In the case of polymer composites with good thermal conductivity but electrical insulation, one should also take into account the tool wear caused by ceramic fillers during production. MgO and Al 2 O 3 fillers, which increase thermal conductivity the most, also cause significant tool abrasion. The optimal filler material should increase thermal conductivity while causing less tool wear. Taking this into consideration, a process efficiency factor is defined as:   www.nature.com/scientificreports/ where φ and ψ are thermal and wear rate enhancement factors, respectively. A high value of for Ω factor means that the filler not only increases heat conduction, but also does not cause too much tool wear. As shown in Fig. 11, natural zeolite is more effective in increasing the thermal conductivity of the composite with less tool wear compared to the other filler contents. The use of hybrid filler is also preferable to Al 2 O 3 , Fritt1-2, or MgO.

Conclusions
The thermally conductive and electrical insulating PA6 composites contain Al 2 O 3 , MgO, Fritt1, Fritt2 glasses, natural zeolite particles, and MgO-natural zeolite hybrid as fillers were successfully fabricated by injection moulding. The following conclusions are drawn: • Each filler modified the crystallisation and melting of PA6.
• The coefficient of thermal conductivity varied from 0.53 to 1.2 depending on the quality and quantity of the filler, which is increasing by from 8 to 144% in comparison to the pure PA6. • The thermal conductivity enhancement factor is highest for Al 2 O 3 (145%), and MgO-zeolite hybrid fillers (92%). • The Lewis-Nielsen and Reciprocity models gave the best approximations to the experimental results. The difference was less than 26%, except for the MgO-loaded composites. • A new process efficiency factor (Ω) was developed that takes into account the manufacturability. A high value of the Ω factor means: not only does the filler increase heat conduction, but it also does not cause too much tool wear. • Considering tool wear caused by fillers, natural zeolite and MgO-natural zeolite hybrid filler are much preferable compared to Al 2 O 3 , Fritt1-2 glass, or MgO. Among heat-conducting composites, the use of hydrosilicate raw materials, which can be mined in large quantities, can reduce the energy demand for the production of the composite.

Data availability
The datasets used and analysed during the current study available from the corresponding author on reasonable request.